Method and apparatus for entrenching submerged elongate structures

ABSTRACT

A method and apparatus for entrenching submerged elongate structures within the bed of a body of water including a floating vessel, and at least one submergible vehicle connected to the floating vessel and operable to be lowered adjacent to a submerged elongate structure to be entrenched. The submergible vehicle includes a frame having a fore end and an aft end with a first primary and a first alternate cutting means connected to the frame and generally operably directed toward the fore end thereof for cutting a trench within the water bed beneath the submerged elongate structure. Additionally, the submergible vehicle may be provided with a second primary and a second alternate cutting means connected to the frame and generally operably directed toward the aft end thereof for forming a trench within the water bed beneath the submerged elongate structure to be entrenched. In one embodiment of the invention a motion compensation system is connected between the floating vessel and at least one submergible vehicle in order to minimize relative, sea induced, motion between the floating vessel and the submergible vehicle.

United States Patent [191 Good et al.

[ 1 Jan. 22, 1974 METHOD AND APPARATUS FOR ENTRENCHING SUBMERGED ELONGATE STRUCTURES [75] Inventors: Alan E. Good; Delbert R. Ward,

both of Houston, Tex.

[73] Assignee: Brown & Root, Inc., Houston, Tex. 22 Filed: May 16, 1972 [21] Appl. No.: 253,895

Primary Examinerlacob Shapiro Attorney, Agent, or F irm- Burns, Doane, Swecker & Mathis [57] ABSTRACT A method and apparatus for entrenching submerged elongate structures within the bed of a body of water including a floating vessel, and at least one submergible vehicle connected to the floating vessel and operable to be lowered adjacent to a submerged elongate structure to be entrenched. The submergible vehicle includes a frame having a fore end and an aft end with a first primary and a first alternate cutting means connected to the frame and generally operably directed toward the fore end thereof for cutting a trench within the water bed beneath the submerged elongate structure. Additionally, the submergible vehicle may be provided with a second primary and a second alternate cutting means connected to the frame and generally operably directed toward the aft end thereof for forming a trench within the water bed beneath the submerged elongate structure to be entrenched.

in one embodiment of the invention a motion compensation system is connected between the floating vessel and at least one submergible vehicle in order to minimize relative, sea induced, motion between the floating vessel and the submergible vehicle.

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SHEET 08 OF H METHOD AND APPARATUS FOR ENTRENCHING SUBMERGED ELONGATE STRUCTURES BACKGROUND OF THE INVENTION This invention pertains generally to a method and apparatus for entrenching submerged elongate structures. More particularly, the invention relates to a method and apparatus for burying submerged pipelines and the like within the bed of a body of water.

With the discovery of large oil and natural gas deposits offshore and the subsequent successful drilling and production thereof, a problem arose in connection with the most economical means for transporting the crude petroleum and/or natural gas from a producing offshore site to a collection or transfer terminal. Often the most economical means for transporting oil and gas offshore has been to establish submerged pipelines extending between the producing and the collecting locations. In this connection U.S. Hauber et al. Pat. Nos. 3,280,571; Lawrence, Pat. Nos. 3,390,532; 3,472,034; and 3,487,648; and Rochelle et al. Pat. No. 3,507,126, all assigned to the assignee of the subject invention, disclose highly effective methods and apparatus for laying pipeline upon the bed of a body of water.

Shortly, however, following the realization that pipelines could be economically laid upon the bed of a body of water, significant problems were encountered, such as' for example, shifting of the pipelines due to currents, corrosion of exposed portions of the pipelines, general structural damage to the pipelines caused by being fouled with anchors, fish nets and similar equipment, etc. In order to obviate or minimize the above noted difficulties it rapidly became common practice to bury or entrench the submerged pipeline structures beneath the surface of the water bed.

I In this connection a number of pipeline burying or entrenching techniques have been proposed and/or utilized with varying degrees of success. More particularly stated, various forms of plows, mechanical rotary cutters, and high pressure fluid jet entrenching systems have at least been proposed in order to bury submerged pipelines.

One particularly advantageous system that has achieved a singular degree of commercial applicationcomprises the utilization of a burying barge adapted to float upon the surface of a body of water and support from the stern thereof a submergible vehicle. The submergible vehicle includes port and starboard pontoon runners which are interconnected by a bridging frame structure. Connected to the bridging frame are downwardly depending port and starboard fluid jet cutting conduits having a plurality of apertures therein for directing high pressure jets of fluid into the water bed.

The submergible vehicle with the fluid jetting conduits attached is adapted to straddle and ride upon a submerged conduit to be entrenched and operatively erode away a trench beneath the conduit by application of the high pressure fluid jets. Once the trench is established, the pipeline descends by gravity beneath the water bed surface. For an illustrative example of such an apparatus reference may be had to FIGS. 1-4 of a US. Tittle Pat. No. 3,338,059, assigned to the assignee of the subject invention.

While the above alluded to burying devices, and particularly the burying device as illustrated in the Tittle patent, are highly advantageous, room for significant structural and operational improvement remains. In this connection previously known devices which have been extremely useful in burying pipelines through normal water bed formations have exhibited a degree of inefficiency in forming a trench through regions or ridges of hard or tough clay formations.

Further, previously known trenching devices have encountered difficulty in rapidly forming a trench beneath pipeline obstructions such as cross-over locations, valve locations, and beneath pipeline risers. In this connection, previous devices have dictated tha such trenching operations be performed by divers utilizing hand jetting equipment. Hand jetting, however, is dangerous and time consuming and thus highly expensive to perform.

Still further while some systems are particularly useful in open field trenching operations difficulties are often encountered in congested field locations such as around an offshore producing tower.

In addition a degree of handling difficulty has been exhibited with previously known systems caused by sea induced motion as a submergible vehicle is being set over a pipeline to be entrenched.

It would therefore be highly desirable to provide a method and apparatus for entrenching submerged elongate structures which would be capable of more efficient trenching operations through hard or tough water bed formations. Further, it would be desirable to provide a method and apparatus for entrenching beneath and around obstructions such as pipeline crossovers, valve stations, and the bottom of risers. Still further it would be desirable to provide a system for efficiently handling trenching operations in both open and congested field locations.

OBJECTS AND SUMMARY OF THE INVENTION Objects It is a general object of the invention to provide a method and apparatus for entrenching submerged elongate structures which will obviate or minimize problems of the type previously described.

It is a particular object of the invention to provide a novel method and apparatus for entrenching submerged elongate structures which will be suitable for use in water bed formations of hard or tough clay.

It is another object of the invention to provide a novel method and apparatus for entrenching submerged elongate structures whichwill be suitable for forming a trench beneath crossing locations of a pipeline to be entrenched and another pipeline.

It is yet another object of the invention to provide a novel method and apparatus for entrenching beneath a valve well positioned within a segment of a submerged pipeline to be entrenched.

It is still another object of the invention to provide a novel method and apparatus for entrenching beneath a bottom portion of a pipeline riser positioned adjacent an offshore tower or the like.

It is a further specific object of the invention to provide a novel method and apparatus for entrenching submerged elongate structures which will minimize relative sea induced motion between a floating vessel and a submergible vehicle suspended from the floating vessel.

It is yet a further object of the invention to provide a novel method and apparatus for entrenching submerged elongate structures which will minimize or eliminate hand jetting requirements around and under pipeline cross-overs, valve locations and at pipeline risers.

It is a still further object of the invention to provide a novel methodand apparatus for entrenching submerged elongate structures for burying pipeline in a relatively open field portion of a water bed and also burying a pipeline in a relatively congested field portion of a water bed such as at the bottom of a pipeline riser and around a plurality of offshore drilling sites where multiple pipeline cross-overs are likely to be encountered. Brief Summary An apparatus suitable to accomplish at least some of the foregoing objects comprises a burying sled or submergible vehicle having a fore and aft end and being operable to be lowered adjacent to and transported along a submerged elongate means, such as a pipeline, to be entrenched. Primary fluid jet cutting means are connected to the burying sled and include a pair of tubular conduits operable to be positioned upon opposite sides of a vertical plane intersecting the longitudinal axis of the submerged pipeline. The tubular conduits are provided with a plurality of forwardly directed jetting apertures for issuing fluid jets into the water bed to form a trench beneath the pipeline to be buried.

An alternate fluid jet cutting means is also connected to the burying sled and includes a pair of tubular conduits having at least one forwardly directed fluid jet provided in each conduit for directing at least one high pressure fluid jet into the water bed on opposite sides of a vertical plane extending through the longitudinal axes of the submerged pipeline. Means are connected to the primary and the alternate fluid jet cutting means for selectively actuating one or the other of said primary or said alternate flud jet cutting means as the circumstances dictate.

A process intended to accomplish at least some of the foregoing objects include utilization of a burying sled as above described with primary and alternate fluid jet cutting means and includes the steps of lowering the submergible vehicle of burying sled adjacent to a pipeline to be entrenched and selectively actuating one or the other of said primary or said alternate cutting means for eroding away a portion of the water bed positioned beneath the submerged elongate conduit.

THE DRAWINGS Other objects and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjection with the accompanying drawings, wherein:

FIG. 1 is a perspective view of an apparatus for entrenching a submerged pipeline including a floating vessel, a submergible vehicle operable to be lowered adjacent to and straddle a submerged conduit to be entrenched;

FIG. 2 is a schematic illustration of a motion compensation system intended to minimize relative seainduced motion between the submergible sled and the floating vessel as depicted in FIG. 1;

FIG. 3 is a detailed side elevational view of a main submergible vehicle having a fore and aft and including a primary fluid jet cutting means directed toward the fore end thereof;

FIG. 4 is a cross-sectional view taken along section line 4-4 of FIG. 3 disclosing a horizontal and vertical adjustment frame of the submergible vehicle;

FIG. 5 is a cross-sectional view of a horizontal adjustment clamp taken along section line 55 of FIG. 4;

FIG. 6 is a front end view of the submergible vehicle disclosed in FIG. 3;

FIG. 7 is a side elevational view of the burying sled disclosed in FIG. 3 particularly illustrating a primary fluid jet cutting conduit directed toward a fore end of the submergible vehicle, a first alternate fluid jet cutting conduit directed toward the fore end of the submergible vehicle and a second alternate fluid cutting conduit directed toward an aft end of the submergible vehicle;

FIG. 8 is a top schematic view of a submergible vehicle as illustrated in FIGS. 3-7 and including a pair of forwardly directed primary fluid jet cutting conduits, a pair of forwardly directed first alternate fluid jet cutting conduits, and a pair of rearwardly directed second alternate fluid jet cutting conduits;

FIG. 9 is a schematic illustrating the utilization of a first alternate fluid jet cutting means to entrench beneath a cross-over section of a conduit to be entrenched;

FIG. 10 is a schematic illustrating the utilization of a second alternate fluid jet cutting means to entrench beneath a cross-over section of a conduit to be entrenched;

FIG. 11 is a perspective view of an apparatus, for entrenching the bottom leg of a pipeline riser, and a pipeline in a congested field such as around a production site, utilizing a submergible burying sled supported from the bow of a floating vessel;

FIG. 12 is an end view of an auxiliary submergible vehicle as disclosed in FIG. 11;

FIG. 13 is a side sectional view taken along section line l3l3 in FIG. 12;

FIG. 14 is a partial sectional view taken along section line 14-14 in FIG. 12, disclosing a horizontal and vertical adjustment frame of the auxiliary submergible sled;

FIG. 15 is a perspective view of a lower segment of the starboard side of the trenching apparatus disclosed in FIG. 12 and particularly illustrates first and second primary fluid jet cutting means, first and second alternate fluid jet cutting means, a central spoil removal conduit and a peripheral fluid plenum chamber surrounding the removal conduit;

FIG. 16 is a cross-sectional view taken along section line 1616 in FIG. 13 and discloses first and second primary fluid jetting conduits, first and second alternate fluid jetting conduits and a central spoil eduction conduit;

FIG. 17 is a schematic front view of the burying sled disclosed in FIGS. 12 and 13 particularly illustrating a first alternate fluid jet cutting conduit system;

FIG. 18 is a sectional view taken along section line 18-18 in FIG. 17 and discloses a valve arrangement for a first and second primary fluid jet cutting means and a first and second alternate fluid jet cutting means; and

FIG. 19 is a top schematic view of the auxiliary burying sled particularly disclosed in FIGS. 12-18 including a schematic illustration of the application of first and second alternate fluid jets.

GENERAL DESCRIPTION General Structure, Open Field Burying Referring now to the drawings, and more particularly to FIG. 1 thereof, there will be seen an elongate structure 20, such as for example a pipeline, cable or the like, laying upon the bed 22 of a body of water 24. It is often desirable, as previously mentioned, to entrench the pipeline within the water bed. Apparatus to entrench the pipeline according to a preferred embodiment of the invention includes a floating marine vessel 30 such as a burying barge, a main submergible vehicle or burying sled 32 and an auxiliary submergible vehicle or burying sled 34.

The main or relatively large burying sled 32 is preferably utilized for open field burying while the auxiliary relatively small burying sled 34 is particular useful in congested fields or areas where frequent crossing lines, tap off valves, etc. are likely to be encountered.

The floating marine vessel or burying barge 30 is a multi-deck structure with essential operational and power generation equipment mounted below, with the usual complement of working equipment above deck, such as for example a deck crane 36, a control bridge 38, storage and personnel quarters 40, a heliport 42,

etc.

The main burying sled 32 includes port and starboard pontoon runners 44 and 46 interconnected by a structural bridge 48. The bridge 48 is designed to support port and starboard cutter and eductor head assemblies 50 and 52. u

The burying sled 32 is suspended from the stern of the bury barge 30 by a block and tackle 58. More particularly a hook on a traveling end of the block and tackle is connected to a four rope bridle 59 which in turn is fastened to the structural framework 48 of the burying sled 32. The upper end of the block and tackle 58 is designed with a nontraveling sheave mounted within the apex 66 of a barge mounted A-frame 62. A dead end of the block and tackle line is connected to a fixed location on board the burying barge through a motion compensation system which will be discussed in detail hereinafter. The live end of the block and-tackle line is connected to a first or forward spool 63 of a three spool deck winch 64. Therefore, actuation of spool 63 will serve to raise and lower the main burying sled 32 as desired.

The submergible vehicle 32 is towed along the pipeline to be entrenched by the provision of a bridle 54 which is connected to the sled pontoon runners and a towing cable 56 which is connected to the bow of the barge 30 and forms a trench beneath the pipeline 20 by jetting high pressure fluid into the water bed through fluid jetting conduits and withdrawing the spoil or detritus material from within the trench by eduction con= duits.

In this connection, high pressure fluid pumps within the hull of the burying barge 30 pump fluid through a high pressure section 80 of a wishbone shaped ladder 82 and down high pressure fluid lines 86 and 88 to the port and starboard cutting heads respectively. The fluid jets slide into the water bed fragmenting and eroding away the earth formation beneath the submerged conduit to be entrenched. In order to remove the thus formed detritus or spoil from the trench a suction pump is provided within the interior of the burying barge. The inlet side of the pump is connected through an eduction section 90 of the wishbone shaped ladder 82 and suction lines 94 and 96 to the port and starboard eduction heads respectively of the burying sled. The outlet side of the pump vents through a spoil conduit 98 to the sea.

In order to facilitate transportation between working sites and/or utilization of the auxiliary burying sled 34 both the A-frame 62 and wishbone shaped ladder 82 are pivotally mounted at locations 70 and 72 to the stern of the barge 30. To facilitate pivotal motion of the A-frame wire rope 74 extends from the apex 66 of the A-frame down to a second or middle spool 76 of the three spool winch 64. In a similar manner a wire rope 77 is attached to the wishbone shaped ladder 82, guided over a pulley mounted at the apex of the A- frame and connected to the traveling end of a block and tackle 78. The nontraveling end of the block and tackle is attached to a king post 79 and the free end of the block and tackle cable is wrapped around a third spool 81 of the three spool winch 64. Therefore controlled actuation of winch spools 76 and 81 will serve to pivot upwardly the A-frame 62 and wishbone ladder In brief summary, in order to entrench a submerged conduit 20 within the water bed in an open field location the A-frame and wishbone are lowered into a posture as generally depicted in FIG. 1. The main burying sled 32 is then lowered onto the pipeline. High pressure fluid such as water under compression is delivered to the cutting head conduits connected to the burying sled 32. Fluid spews in jets into the water bed to form a trench beneath the conduit to be buried. Spoil or detritus is simultaneously removed from the immediate vicinity of the pipeline by the provision of a suction system which sucks the spoil up into the barge hull and dumps the spoil over board through a conduit 98. As the barge 30 is winched along the pipeline route the sled 32 will thus form a trench that the pipeline may descend into. This trench will eventually be covered or backfilled by sea currents.

Motion Compensation System Rough seas sometimes tend to pitch the floating barge about an axis extending horizontally through the center of the barge and normal to the port and starboard side thereof. This pitching motion will raise and lower the stern of the barge which in turn will directly induce a corresponding raising and lowering movement of the suspended main sled 32. Such motion is particularly undesirable when the sled is raised and lowered onto a pipeline since divers are frequently used to hand guide the sled into position.

Turning now to FIG. 2 there will be seen schematically illustrated a motion compensation system suitable to minimize relative motion between the floating pipelaying barge 30 and the submergible burying sled 32 caused by variable hydrodynamic forces of the sea. As previously noted, the wire rope or cable 58 which is connected to the submergible burying sled 32 is also connected at the upper end thereof to a traveling sheave block 1102 by a suitable hook connector 104. A plurality of pulleys or sheaves 106 are also mounted at the apex 66 of the A-frame 62. Reaved around the sheaves 106 and 102 is a wire rope or cable 108. The live end of cable 108 is connected to a spool 63 of a three spool winch 64 which is mounted upon the deck 110 of the burying barge 30. The other end of the wire cable 108 is connected through a motion compensation unit 112 to a dead mount 114 fixedly connected to the barge 110.

The motion compensation device 112 includes first and second sheave blocks 116 and 118, respectively, which are interconnected by a variable displacement cylinder 120. In this connection the sheave block 118 is free to translate or travel while the sheave block 116 is fixedly mounted on the deck 110.

The variable displacement cylinder 120 is designed to expand and contract and thus take up and pay out line with a constant tension on the cable 108 in order to compensate for relative sea induced motion between the hanging submergible sled 32 and the burying barge 30.

The actual motion compensation mechanism 112 per se does not form a part of the subject invention and conventional pneumatic or hydraulic units may be utilized. At least one motion compensation unit suitable for use with the subject invention is disclosed in US. PrudHomm et al. Pat. No. 3,314,657. The relevant disclosure of the Prudl-Iomme et al. patent is hereby incorporated by reference as though set forth at length. Detailed Structure Main Sled Turning now with particular attention to FIGS. 3-7 there will be seen detailed structural views of the main burying sled 32 of the subject invention.

As particularly illustrated in FIGS. 3 and 6, it will be seen that the burying sled 32 is fashioned having port and starboard tubular pontoon runners 44 and 46 respectively. Each of these tubular pontoon runners is provided with a weld cap 154 at the aft end thereof and a sloping face plate 156 at the fore end, thus enclosing the pontoon to form a watertight buoyancy chamber for facilitating manipulation of the burying sled while in a submerged posture. In this connection, the buoyancy of the sled may be regulated by the provision of vent 155 and ballast 157 valves in valve wells 158, as best illustrated in FIG. 8.

The port and starboard pontoon runners are further provided at the fore ends thereof with sloping towing eyes 180 to enable the bridle 54 to be readily connected to the submergible burying sled 32.

As previously noted, the pontoon runners are interconnected by a bridging framework 48. In this connection port pontoon runner 44, as illustrated in FIG. 8, is provided with a fore mounted upstanding support column 160, a central support column 162 and an aft support column 164. In like manner, the starboard pontoon runner 46 is provided with a fore mounted upstanding column 166, a central column 168 and an aft column 170. Fixedly attached to the top of upright colomns 160, 164, 166 and 170 are normally extending pad eyes 178 which provide a ready source of connection for the four member bridle 59. The central columns 162 and 158 are interconnected by a horizontally extending bridging support member 172. In a like manner, the aft upright columns 164 and 170 are interconnected by a horizontally spanning support member 174.

The upright columns and horizontally extending brace members form the basic elements of the bridging framework 48. These basic framework elements are structurally ruggedized by the provision of a plurality of bracing members 176.

The horizontal spanning member 172 is provided with port and starboard vertical adjustment columns 182 and 184, note FIG. 4, which extend normally with respect thereto. Each of the adjustment columns are provided with rearwardly directed vertical support rails 186 having a plurality of adjustment apertures 188 fashioned therethrough. In a similar manner the horizontal spanning member 174 is provided with port and starboard vertical adjustment columns 190 and 192. The adjustment columns 190 and 192 each are provided with T-slot guide rails 194 having a plurality of apertures 196 fashioned therethrough.

The columns 190 and 192 are provided with structural support by the provision of struts 198 and 200, note FIGS. 5 and 8, which extend from the port and starboard pontoon runners up to an upper portion of the support columns 190 and 192, respectively.

Mounted for horizontal and vertical adjustment capability within the above described bridging frame 48 are port and starboard cutter and eductor head assemblies 50 and 52. In this connection the cutter and eductor had assemblies 50 and 52 are connected to port and starboard lateral arms of a box frame assembly 250, note FIG. 4. The lateral arms are connected to translate horizontally on fore and aft transversely extending arms which in turn are mounted at the ends thereof for vertical adjustment upon columns 182, 184, 190 and 192.

More particularly, the cutter and eductor head assemblies 50 and 52 are normally fashioned through longitudinally extending support arms 260 and 268 of the box frame 250. The longitudinal support arms 260 and 268 are fashioned at the fore and'aft ends thereof with horizontal adjustment bracket assemblies 270 and 290, respectively. The bracket assembly slidingly encompass fore 280 and aft 296 transversely extending support arms of the box frame 250.

Each of the bracket assemblies 270 and 290 comprise opposing C-shaped pads 272 and 274. The pads are provided with normally extending apertured flanges 276 at the edges thereof to receive threaded fasteners 294 or the like to clamp the assemblies to the support arms. To provide for maximum position integrity upper and lower rails 291 and 292 may be connected onto the support arms. The rails are provided with a plurality of apertures (not shown) to align with apertures in the flanges 276 of the bracket assemblies. Therefore the threaded fasteners 294 may be used to rigidly join the horizontal adjustment assemblies with the normally extending support arms 280 and 296.

The ends of the transerve adjustment bar 280 are provided with brackets 292 which support cantilever runners 284. The runners 284 are intimately received within the vertically extending adjustment rails 186. At least one aperture 285, note FIG. 7, is provided with the cantilever arms 284 so that a bolt 286 may be extended through an aperture 188 in the guide rails 186 through the cantilever arms 284 to provide selective vertical adjustment of the transverse bar 280.

In a similar manner the ends of the transverse adjustment bar 296 are provided with brackets 298 which carry cantilever T-shaped runners 300. The T-shaped runners 300 are designed to intimately engage with the T-shaped guide rails 194 and are provided with at least one aperture 301 for alignment with apertures 196 in the guide rails 194. A bolt 302 may then be extended through an aperture 196 in the guide rails and through the apertures 301 in the cantilever arms 300 to maintain a vertical adjustment of the transverse member 296.

From the foregoing it will be seen that the port and starboard cutting and eductor heads 50 and 52 are mounted for selective horizontal and vertical adjustment with respect to the bridging frame 48 of the submergible burying sled 32. Cutter and Eductor Head Assemblies Main Sled As particularly illustrated in FIGS. 3, 6 and 7, the

port and starboard cutter and eductor head assemblies 50 and 52 are each provided with an eduction conduit 350. As previously discussed, the eduction conduits 350 are connected to a suction pump on board barge 30 and are designed to extend within a trench and remove spoil or detritus as the cutting head fashions a trench within the water bed.

Attached to the fore and aft faces of the conduit 350 are fore and aft guide rollers 352 and 354 respectively, which are designed to detect the presence of a conduit to be buried so that the sled may be maintained in a symmetric straddling posture over the conduit. Further, a horizontally extending top roller 356 is connected to the horizontal spanning members 172 and 174 to detect the proximity of the horizontal members with respect to the burying sled.

Turning now particularly to FIG. 7 there will be seen a detailed side elevational view of the starboard cutting and eduction head assembly 52 of the main burying sled 32. The assembly includes a primary cutting head conduit 300 having a plurality of apertures 362 fashioned therein which open generally toward the fore end of the burying sled for directing high pressure of jets of fluid into the water bed 22 to be cut away. As previously noted, the high pressure fluid is delivered to the starboard cutting head 30 through a high pressure fluid supply line 88 which is connected to a compressor mounted within the hull of the burying barge 30.

Normally tapped into a section 364 of the starboard primary cutter head conduit 300 is a first alternate cutting head 366. The first alternate cutting head conduit 366 extends generally coextensive and mutually parallel with the primary fluid jet cutting conduit 360 and may be provided with one or more apertures for directing fluid in a direction toward the fore end of the burying sled 32.

In a preferred embodiment as particularly illustrated in FIGS. 7 and 8, the first alternate cutting head 366 terminates in a single jetting head nozzle 368 which is suitable to direct an extremely high pressure fluid jet into the water bed.

While it is preferred that the first alternate fluid jet cutting head be provided with a single fluid nozzle, a plurality of apertures, as noted above, are contemplated by the subject invention with the limited qualification that the total number of apertures in an alternate fluid jet cutting conduit shall be less than the number of apertures or nozzles in a corresponding primary fluid jet cutting conduit. Therefore, for an equal volume of fluid delivered from the high pressure pump, stronger jets will be realized, even considering losses, from an alternate fluid jet cutting head than a corresponding primary fluid jet cutting head.

In like manner, note FIG. 8, the port cutting and eduction head assembly 50 is provided with a primary fluid jet cutting conduit 370 having a plurality of forwardly directed apertures fashioned therein. The primary fluid jet cutting conduit 370 is fed by a high pressure line 86 which is also connected to a compressor within the interior of the burying barge 30. As described in connection with the starboard primary cutter head, a first alternate fluid jet cutting head 372 is tapped off of primary port cutting head 370. The first alternate cutting head 372 extends generally coextensively and mutually parallel with the port primary cutter head and preferably terminates, as previously discussed, in a single fluid jet nozzle 374.

As particularly illustrated in FIG. 8, the port and starboard first alternate fluid jet cutting heads 372 and 366 are angled inwardly with respect to each other so that high pressure fluid jets emanating therefrom 375 and 369 will intersect at approximately the intersection of a first vertical plane extending through the longitudinal axis of a conduit to be buried and a second vertical plane extending normally to the first vertical plane and passing through the fore end of the burying sled.

Also normally tapped into a section 364 of the starboard primary fluid jet cutter head conduit 360 is a second alternate fluid jet cutting conduit 380 which extends downwardly along the eduction conduit 350 and preferably, as previously noted, terminates in a single nozzle 382 directed toward an aft end of the submergible burying sled 32. In a similar manner a second alternate fluid jet cutting conduit 384 is tapped into the port primary fluid jet cutting conduit 370. The port side second alternate fluid jet cutting conduit also preferably terminates in a single nozzle 386 directed toward an aft end of the burying sled.

As with the first alternate port and starboard fluid jet conduits the second alternate port and starboard fluid jet cutting conduits are generally angled together so that very high pressure fluid jets emanating therefrom 387 and 383 will intersect at approximately the intersection of a first vertical plane extending through the longitudinal axis of a conduit to be buried and a second vertical plane extending normally to the first vertical plane and passing through the aft end of the burying sled.

A particularly significant aspect of the subject invention is the above described provision of the primary fluid jet cutting conduits in conjunction with the first and second alternate fluid jet cutting conduits. Additionally significant, however, is the provision for selective control or actuation of each of these conduits.

In this connection, the starboard primary fluid jet cutting conduit 360 is provided with a ball valve 400 within a section 402 thereof which is downstream of previously mentioned section 364. The first alternate fluid jet cutting member 386 is provided with a ball valve 404 and the second alternate fluid jet cutting member 380 is provided with a ball valve 406.

in a similar manner the port primary fluid jet cutting conduit 370 is provided with a similar ball valve 408 downstream of the tap off location (not shown) of the first and second alternate fluid jet cutting conduits. The port first and second alternate fluid jet cutting conduits 372 and 384 respectively are each provided with a ball valve (not shown) similar to ball valves 404 and 406 in the starboard first and second alternate fluid jet cutting conduits.

Each of the above described ball valves is preferably designed to be manually controllable by a diver. It will be appreciated, however, that other particular valve structures and means of actuation are fully contemplated by the subject invention.

Process of Utilizing Main Sled With reference now to FIG. 1 and as previously discussed, for burying a pipeline in an open field the main burying sled 32 is lowered over the pipeline onto the seabed. The primary fluid jet cutting heads are actuated by opening ball valves 400 and 408 and closing the ball valves in the first and second alternate fluid jet cutting head lines. High pressure jets of fluid will then slice and cut into the water bed formation and the spoil or detritus produced by such cutting action will be withdrawn from beneath the pipeline by suction through the eductor heads. Therefore as the barge 30 pulls the sled along the pipeline route a trench will be fashioned within the water bed.

Upon encountering, however, water bed formations of tough or hard clay or coral, such as ridges 400, it is necessary, in order to proceed in an efficient manner, to increase the cutting action of the fluid jet. This may be achieved by closing off ball valves 400 and 408 in the primary fluid jet cutting conduits and directing all of the pressurized fluid through the first alternate fluid jet cutting conduits 366 and 372 by opening ball valves in the first alternate fluid jet conduit lines. Therefore, streams of intensified pressurized fluid will slice into the hard or tough water bed to break up and crumble these formations. In the event that this breaking up procedure does not sufficiently fragmentize the formation to permit ready eduction, selective alternate applications of the primary and first alternate fluid jet cutting means may be utilized to break the formation into a convenient size for removal.

In at least one further mode of operation, the subject main sled cutting head system has been particularly useful. In this connection and with reference to FIGS. 9 and 10, it will be seen that a conduit to be buried 20 may cross beneath a subsequently laid conduit 402. In trenching up to the crossing conduit 402 the first and second alternate fluid jet cutting ball valves are closed and the primary fluid jet cutting means is utilized to form a trench beneath the conduit 20 as previously discussed. Upon reaching the crossing conduit 402 it will be appreciated that the fore portion of the pontoon runners will tend to engage the crossing conduit 402 and prevent full removal of soil beneath the conduit 20 at the crossing junction.

In order to minimize a requirement for hand jetting techniques in this situation, the primary fluid jet ball valves 400 and 408 are closed and the first alternate fluid jet ball valves are opened to allow intensified or strong jets of fluid to issue from the first alternate fluid jet conduits 366 and 372 toward the fore end of the pontoon runners, note FIG. 8, and beneath the crossing junction.

Upon flushing away of the material on the approach side of the crossing junction, all of the valves in the fluid conduits are closed and the sled is lifted up by actuation of the first spool 63 of the three spool winch 64 and then lowered again onto the seabed on the other side of the crossing junction, as particularly illustrated in FIG. 10.

The second alternate rearwardly directed fluid jet cutting conduits 382 and 386 are then actuated by opening the ball valves connected within the second alternate fluid jet cutting conduits to enable intensified or strong jets of fluid to complete the flushing away process beneath the cross-over location and permit the pipeline 20 to descend within the water bed.

Following the removal of the seabed from beneath the conduit 20 at the crossing junction, the second alternate fluid jet cutting conduits are deactuated by closing the ball valves therein and the primary fluid jet cutting conduits 360 and 370 are reactuated by opening ball valves 400 and 408, respectively. The sled is then towed along the pipeline route to entrench the pipeline as previously discussed.

General Structure, Congested Field Burying With particular reference now to FIG. 11, it will be seen that the floating burying barge 30 is depicted in a position adjacent to an offshore tower 500.

The offshore tower 500 includes a base portion 502 which rests upon and extends into the water bed 22 with an upper portion 504 which extends above the surface of the body of water 24 to support a drilling platform and/or a production platform and/or a storage facility, etc. Fluid communication between the submerged elongate means 20 to be entrenched and the platform is achieved by the provision of a generally L- shaped conduit 506 which is commonly termed a pipeline riser.

The pipeline riser 506 is clamped for support to a leg of the offshore tower 500 by the provision of a plurality of adjustable clamping assemblies. For a detailed disclosure of a clamping assembly suitable for attaching a riser to an offshore tower, reference may be had to a US. I-Iauber Pat. No. 3,557,564, assigned to the assignee of the present invention.

Once the pipeline 20 has been laid adjacent to an offshore tower with a riser 506 connected thereto, a problem has been encountered in the past in connection with an advantageous method and apparatus for burying the lower leg of the riser at the bend section 505. Moreover, a problem also exists in connection with burying the pipeline 20 in locations generally adjacent an offshore tower 500 because of the normally congested area created by frequent crossing of other pipelines, such as 508 and 510 and/or the provision of tap off valves 511.

In order to bury the bottom section 505 of the riser 506 and the conduit 20 around the offshore tower 500, the previously mentioned auxiliary submergible burying sled 34 is utilized. In this connection, the main burying sled 32 is raised by actuation of the first spool 63 of the three spool winch 64 and mounted between the legs of the A-frame 62 on the barge stern. Also the wishbone ladder 82 and the A-frame 62 are pivoted into a raised posture by actuation of the second and third spools 76 and 81 of the three spool winch.

The barge is then maneuvered to a position where the bow thereof is adjacent to the riser 506 connected to the offshore tower 500. The deck crane 36 is then utilized to lower the auxiliary burying sled 34 onto the bottom bend 505 of the pipeline riser 506 with a fore portion of the submergible sled 34 directed away from the tower 500. A bridle 512 may be attached to the sled 34 for connection to a towing wire rope 514 which in turn is attached to the stern of the burying barge 30. Thus advancement of the auxiliary submergible sled 34 away from the pipeline riser 506 is achieved by winching the burying barge 30 backwards or in a mode of operation with the stern proceeding the bow in the direction of travel. A detailed operational description of the auxiliary sled 34 will be discussed hereinafter following a structural description thereof.

Detailed Structure Auxiliary Sled Referring now to FIG. 12 there will be seen a front elevational view of the auxiliary sled 34. As with the main sled 32, the auxiliary sled is provided with port and starboard pontoon runners 520 and 522. The pontoon runners are adapted to rest upon and slide along the surface of the water bed 22. Each of the pontoon runners is provided with a normally extending pair of vertical columns 524, note FIGS. 12 and 14. The columns 524 serve to support a fore and aft bridging frame member 526 and 528. The horizontal bridging frame members 526 and 528 are in turn supported by sloping struts 534 which extend between the horizontal members and the port and starboard pontoons to lend structural rigidily to the bridging framework.

The bridging members 526 and 528 are each provided with port and starboard upright vertical adjustment columns 530 and 532. The port side fore and aft vertical adjustment columns 530 and the starboard side fore and aft vertical adjustment columns 532 are provided with inwardly facing vertical adjustment rails 536. Each of the adjustment rails is provided with a plurality of apertures 538 fashioned therein at regular intervals along the vertical height. I

Mounted for vertical and horizontal adjustment within the above described pontoon framing structure are port and starboard cutter and eductor head assemblies 540 and 542.

The port and starboard cutter and eductor head assemblies' are supported for vertical and horizontal adjustment by a box frame 543, note FIGS. 14 and 19, including fore and aft transversely extending support beams 544 and 546 which arev spanned by transversely adjustable port and starboard longitudinally extending beams 548 and 550. v

The fore and aft transverse arms 544 and 546 are provided at the ends thereof with vertical adjustment bracket members 552 having an aperture therethrough to receive aconventional fastener 554. Therefore the adjustment brackets 552 may be vertically raised and lowered along the adjustment rails 536 until a desired elevation is achieved and the fasteners, such as bolts 554, may then be slid through aligned apertures in the brackets and the vertical guide rails to fix the elevation of the cutter and eductor head assemblies.

Horizontal adjustability of the port and starboard cutter and eductor head assemblies 540 and 542 is achieved by the provision of clamping members 560 positioned at the ends of the port and starboard longitudinal support members 548 and 550. The clamping members 550 releasably embrace the fore and aft transverse arms 544 and 546 in a manner as previously described in connection with the main sled to permit the port and starboard cutter and eductor head assemblies S40 and 542 to be selectively horizontally adjusted.

Cutter and Eductor-I-Iead Assemblies Auxiliary Sled Referring now particularly to FIG. 13, there will be seen a side elevational view of the port cutter and eductor head assembly 540.

The eductor head assembly comprises a lower conduit member 570 having a generally oval cross section which extends upwardly through a reduction section 572 to a generally circular conduit segment 574. The conduit segment 574 terminates in an outwardly directed wing section 576, note FIG. 12.

The lowermost end of the eduction conduit 570 is provided with an inwardly facing cutout portion 578,

note the starboard eduction head illustrated in FIG. 15. The cutout area provides a large entry opening to permit fluidized spoil to readily enter the eduction system. The fluidized spoil is raised up through the eduction conduit 570 by a gaslift technique.

More particularly, high pressure gas is injected into the eduction conduit 570 from a line 601 which is connected to a pump on board the burying barge 30. The submerged end of the line 601 empties into a generally elliptical plenum chamber 582 which surrounds a lower end of the eduction head 570. A plurality of nozzles 584 are mounted within the interior of the plenum 582 and extend through the shell of the eduction conduit 570 to introduce a plurality of fluid jets into the interior of the eduction conduit. The pressurized fluid comingles with the fluidized detritus or spoil within the interior of the eduction conduit to reduce the specific gravity of the mixture and raise the spoil within the eduction head 570 up and out of the uppermost end thereof 576. Therefore detritus or spoil left within a trench fashioned into the water bed by the port cutting head assembly may be removed to permit the pipeline to descend within the trench.

The starboard eduction head assembly structurally is a mirror image-of the port eduction head assembly and functionally serves to remove fluidized'spoil from the trench as fashioned by the starboard cutting head assembly in 'a manner-as discussed above.

The port cutter and eductor head assembly 540 is also provided with a fluid jet cutting system 590 including a first primary fore directed fluid jet cutting conduit 592 and a second primary aft directed fluid jet cutting conduit 594. The first and second primary fluid jet cutting conduits 592 and 594 are each provided with a plurality of generally forwardly and rearwardly directed apertures 596 and 598 therein, respectively.

High pressure fluid is fed into the first and second primary fluid jet cutting conduits 592 and 594 by a high pressure supply line 601 which in turn is connected to a high pressure fluid pump (not shown) within the interior of the burying barge 30, note FIG. 11.

The starboard cutter and eductor head assembly 542 is in a like manner provided with a first primary and a second primary fluid jet cutting conduit 600 and 602,

' respectively, note FIG. 15. The first and second primary fluid jet cutting conduits are provided with a plurality of forwardly and rearwardly directed fluid jetting apertures 604 and 606 respectively for issuing high pressure fluid jet into the water bed to be removed.

The first and second starboard primary fluid jet conduits 600 and 602 are each connected at the upper end thereof to a flexible transverse coupling 608, note FIG. 12, which conduits extend to coupling tees 610 and 611 fashioned respectively in the first and second primary fluid jet cutting conduits of the port fluid jet cutting assembly 540. Therefore, high pressure fluid delivered to the first primary fluid jet cutting conduit on the port side from line 601 will also be simultaneously delivered to the first primary fluid jet cutting conduit on the starboard side of the burying sled. In a like manner the port and starboard second primary fluid jet cutting conduits will be supplied simultaneously with high pressure fluid.

Actuation of the port and starboard first primary fluid jet cutting conduits 592 and 600 is controlled by a ball valve 620 positioned in a conduit segment 622 which interconnects the high pressure fluid line 601 and the coupling tee 610. Therefore, when ball valve 620 is in a closed posture no fluid will be directed from the port and starboard first primary fluid jet cutting conduits. However, upon opening ball valve 620, high pressure fluid jets will issue from the jetting apertures 596 and 604 into the water bed 22 in a direction toward the fore end of the auxiliary burying sled 34.

In a similar manner a ball valve 630 is connected into a connecting line segment 632 which interconnects the high pressure source line 600 and the coupling tee 611. Therefore, when ball valve 630 is in a closed posture no fluid will be directed from the port and starboard second primary fluid jet cutting conduits. However, upon opening ball valve 630, fluid jets will simultaneously issue from the plurality of jetting apertures 598 and 606 into the water bed in a direction toward the aft end of the auxiliary burying sled 34.

The port cutter and eductor head assembly 540, note FIG. 13, is also provided with first and second alternate fluid jet cutting conduits 650 and 652 which tap into branches 622 and 632, respectively, above the ball valves 620 and 630. The first and second fluid jet cutting conduits 650 and 652 extend generally parallel and coextensive with the first and second primary fluid jet cutting conduits 592 and 594, respectively. In a preferred embodiment, the first and second alternate fluid jet cutting conduits terminate in unitary normally extending legs 654 and 656 which are bent around the adjacent primary fluid jetting conduits and point in a direction generally toward the fore and aft ends respectively of the burying sled. The legs 654 and 656 may be provided with nozzles 658 and 660 for directing very high pressure fluid jets into the bed of the body of water in a forward and aft direction respectively.

while unitary jets are preferred, other jetting arrangements are contemplated by the subject invention, with the limitation that the number of jetting nozzles or apertures in an alternate fluid jet cutting conduit does not equal or exceed the number of jetting apertures in a corresponding primary fluid jet cutting conduit.

ln a like manner, the starboard cutter and eductor head assembly 542 is provided with first and second alternate fluid jet cutting conduits 670 and 672, note FIG. 15, which extend generally coextensive with the starboard first and second primary fluid jet conduits 600 and 602 and are provided at the lowermost portion with normally extending legs 674 and 676 which tenninate in nozzles 678 and 680 directed toward the fore and aft ends of the sled respectively. The first alternate starboard fluid jet cutting conduit 670 is connected in fluid communication with the first alternate port fluid jet cutting conduit 650 by the provision of a flexible cross-over 680, note FIG. 17. In a similar manner a flexible cross-over (not shown) interconnects second alternate starboard fluid jet cutting conduit 672 and a second alternate port fluid jet cutting conduit 652.

In order for pressurized fluid to be delivered to the port and starboard first alternate high pressure fluid jet cutting conduits 650 and 670, a line 684 connecting the two is tapped into branch 622 of the first primary fluid jet cutting system above ball valve 620, note FIGS. 13, 17 and 18. Control of fluid through the port and starboard first alternate high pressure fluid jet cutting conduits 650 and 670 is achieved by the provision of a ball valve 682 in the line 684. Therefore, when the ball valve 682 is opened, high pressure fluid will be distributed to the port and starboard first alternate high pressure fluid jet cutting conduits 650 and 670 and high pressure fluid jets will issue from nozzles 658 and 678 respectively into the water bed in a direction toward the fore end of the auxiliary burying sled 34, such as particularly illustrated in FIG. 19.

In a similar manner the port and starboard second alternate high pressure fluid jet cutting conduits 652 and 672 are placed in fluid communication with the high pressure source of fluid by joining into a conduit 690 which is tapped into leg 632 of the second primary fluid jet cutting system upstream of the ball valve 630. In order to control the issuance of high pressure fluid jets from the port and starboard second alternate high pressure fluid jet cutting conduits, a ball valve 692 is positioned within the line 690. Therefore, when ball valve 692 is opened, high pressure will be delivered to the port and starboard second alternate high pressure fluid jet cutting conduits 652 and 672 so that extremely high pressure jets of fluid will issue from nozzles 660 and 680 toward the aft end of the auxiliary burying sled 34, as particularly illustrated in FIG. 19.

Also as illustrated in FIG. 19 the port and starboard first alternate fluid jet cutting conduit nozzles 658 and 678 are angled generally inwardly and are preferably designed to issue unitary fluid jets 710 and 712 which will cross generally at an intersection of a first vertical plane extending vertically through the longitudinal axis of the conduit 20 to be buried and a second vertical plane extending normally to the first plane and through the fore end of the burying sled. In a similar manner, the port and starboard second alternate fluid jet cutting conduit nozzles 660 and 680 are angled generally inwardly and are preferably designed to issue unitary fluid jets 700 and 702 which will cross generally at an intersection of the first vertical plane extending through the longitudinal axis of the conduit 20 to be buried and a third vertical plane extending normally to the first vertical plane and through the aft end of the burying sled.

The above described preferred inclination of the first and second alternate fluid jet cutting nozzles enables the sled to trench beneath obstructions and the like in a manner to be discussed presently in detail.

Process of Utilizing Auxiliary Sled As previously stated, it is in some instances difficult to effectively utilize the main burying sled 32, such as for example for burying the L-section of a pipeline riser, or close in work in a high density field where numerous crossing pipelines, tap off valves, etc., are likely to be encountered. In at least these instances, the provision of an auxiliary sled 34 has proven to be highly advantageous.

In one operational mode, the bow crane 36 serves to lower the auxiliary sled 34 with an aft end thereof abutted against a bend portion 505 of the pipeline riser 506 as particularly illustrated in FIG. 11. High pressure fluid lines 601 and 603 are lowered from the bow. of the floating barge 30 and are connected to the fluid jetting system and the air eduction system of the auxiliary burying sled 34, respectively.

In order to jet away a portion of the water bed 22 beneath the riser bend 505, the ball valves 620 and 630 in the primary fluid jetting cutting system are closed and the ball valve 682 connected to the port and starboard first alternate fluid jet cutting conduits is also closed. The ball valve 692, however, is opened which provides fluid communication from the high pressure 

1. An apparatus for entrenching a submerged elongate means within the bed of a body of water comprising: a frame having a fore end and an aft end operable to be lowered adjacent to and transported along a submerged elongate means to be entrenched; primary fluid jet cutting means connected to said frame and directed toward the free end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched, said primary fluid jet cutting means including, a pair of tubular conduits operable to extend with one of said pair of conduits on one side of a vertical plane extending longitudinally through the submerged elongate means to be entrenched and with the other one of said pair of conduits on an opposite side of the vertical plane extending longitudinally through the submerged elongate means to be entrenched with said pair of tubular conduits having a plurality of apertures therein for directing a plurality of fluid jets generally toward the fore end of said frame and into the water bed on generally opposite sides of the vertical plane extending longitudinally through the submerged elongate means to be entrenched; alternate fluid jet cutting means connected to said frame and directed toward the fore end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched, said alternate fluid jet cutting means including, a pair of tubular conduits operable to extend with one of said pair of conduits on one side of the vertical plane extending longitudinally through the submerged elongate means to be entrenched and with the other one of said pair of conduits on an opposite side of the vertical plane extending longitudinally through the submerged elongate means to be entrenched with each of said tubular conduits having at least one but less than the total number of apertures in a corresponding conduit of said primary fluid jet cutting means for directing one or more fluid jets generally toward the fore end of said frame and said pair of tubular conduits being operable to direct fluid jets into the water bed on generally opposite sides of the vertical plane extending longitudinally through the submerged elongate means to be entrenched; and means connected to said primary and said alternate cutting means for selectively actuating one or the other of said priMary or said alternate cutting means.
 2. An apparatus for entrenching a submerged elongate means as defined in claim 1 wherein: each of said pair of tubular conduits comprising said alternate fluid jet cutting means is provided with a single aperture therein for directing a unitary fluid jet therefrom generally toward the forward end of said frame and into the water bed.
 3. An apparatus for entrenching a submerged elongate means as defined in claim 2 wherein: said apertures in said pair of tubular conduits comprising said alternate fluid jet cutting means are fashioned at a mutually convergent angle such that unitary fluid jets emanating from said apertures will each intersect the vertical plane extending longitudinally through the submerged elongate means at approximately the intersection of said plane and a second vertical plane lying normal to said plane and extending through the fore end of said frame.
 4. An apparatus for entrenching a submerged elongate means as defined in claim 1 wherein said means for selectively actuating comprises: a valve positioned in each conduit of said pair of primary fluid jet tubular conduits; and a valve positioned in each conduit of said pair of alternate fluid jet tubular conduits.
 5. An apparatus for entrenching a submerged elongate means within the bed of a body of water comprising: a frame having a fore end and an aft end operable to be lowered adjacent to and transported along a submerged elongate means to be entrenched; primary fluid jet cutting means connected to said frame and directed toward the fore end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched; first alternate fluid jet cutting means connected to said frame and directed toward the fore end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched; second alternate fluid jet cutting means connected to said frame and directed toward the aft end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched; and means connected to said primary cutting means, said first alternate cutting means and said second alternate cutting means for selectively actuating one of said primary cutting means, said first alternate cutting means, or said second alternate cutting means.
 6. An apparatus for entrenching a sumberged elongate means within the bed of a body of water comprising: a frame having a fore end and an aft end operable to be lowered adjacent to and transported along a submerged elongate means to be entrenched; first primary fluid jet emitting means connected to said frame and directed toward the fore end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched; second primary fluid jet cutting means connected to said frame and directed toward the aft end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched; first alternate fluid jet means connected to said frame and directed toward the fore end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched; second alternate fluid jet cutting means connected to said frame and directed toward the aft end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched; and means connected to said first primary cutting means, said secondary primary cutting means, said first alternate cutting means and second alternate cutting means for selectively actuating one of said first primary cutting means, said second primary cutting means, said first alternate cutting means, or said second alternate cutting means.
 7. An apparatus for entrenching a submerged elongate means as defined in claim 6 wherein said first primary fluid jet cutting means each cOmprise: a pair of tubular conduits operable to extend with one of said pair of conduits on one side of a vertical plane extending longitudinally through the submerged elongate means to be entrenched and with the other one of said pair of conduits on an opposite side of the vertical plane extending longitudinally through the submerged elongate means to be entrenched with said pair of tubular conduits of said first and second primary fluid jet cutting means having a plurality of apertures therein for directing a plurality of fluid jets generally toward the fore and aft ends respectively of said frame and into the water bed on generally opposite sides of the vertical plane extending longitudinally through the submerged elongate means.
 8. An apparatus for entrenching a submerged elongate means as defined in claim 7 wherein said first and second alternate fluid jet cutting means each comprise: a pair of tubular conduits operable to extend with one of said pair of conduits on one side of the vertical plane extending longitudinally through the submerged elongate means to be entrenched and with the other one of said pair of conduits on an opposite side of the vertical plane extending longitudinally through the submerged elongate means to be entrenched with each of said tubular conduits having at least one but less than the total number of apertures in a corresponding conduit of said first and second primary fluid jet cutting means for directing one or more fluid jets generally toward the fore and aft ends respectively of said frame and said pair of tubular conduits being operable to direct fluid jets into the water bed on generally opposite sides of the vertical plane extending longitudinally through the submerged elongate means to be entrenched.
 9. An apparatus for entrenching a submerged elongate means as defined in claim 8 wherein: each of said tubular conduits comprising said first and second alternate fluid jet cutting means is provided with a single aperture therein for directing a unitary fluid jet therefrom generally toward and into the water bed.
 10. An apparatus for entrenching a submerged elongate means as defined in claim 9 wherein: said apertures in said pair of tubular conduits comprising said first alternate fluid jet cutting means are fashioned at a mutually convergent angle such that unitary fluid jets emanating from said apertures will each intersect the vertical plane extending longitudinally through the submerged elongate means at approximately the intersection of said plane and a second vertical plane lying normal to said plane and extending through the fore end of said frame; and said apertures in said pair of tubular conduits comprising said second alternate fluid jet cutting means are fashioned at a mutually convergent angle such that unitary fluid jets emanating from said apertures will each intersect the vertical plane extending longitudinally through the submerged elongate means at approximately the intersection of said plane and a second vertical plane lying normal to said plane and extending through the aft end of said frame.
 11. An apparatus for entrenching a submerged elongate means as defined in claim 6 wherein said means for selectively actuating comprises: a valve positioned in each of said first and second primary and said first and second alternate cutting means for selectively actuating any one of said first and second primary and said first and second alternate cutting means.
 12. An apparatus for entrenching a submerged elongate means within the bed of a body of water comprising: a floating vessel; a submergible vehicle operable to be lowered adjacent to a submerged elongate means to be entrenched, said submergible vehicle including, a frame having a fore end and an aft end; primary fluid jet cutting means connected to said frame and directed toward the fore end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched; first aLternate fluid jet cutting means connected to said frame and directed toward the fore end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched; second alternate fluid jet cutting means connected to said frame and directed toward the aft end thereof for forming a trench within the water bed generally beneath the submerged elongate means to be entrenched; means connecting said fore end of said submergible vehicle with said floating vessel for inducing movement of said submergible vehicle in response to movement of said floating vessel; means connected to said floating vessel for pressurizing fluid; means for connecting in fluid communication said means for pressurizing fluid and said primary fluid jet cutting means, said first alternate fluid jet cutting means and said second alternate fluid jet cutting means; and means connected to said primary, said first alternate and said second alternate fluid jet cutting means for selectively controlling the flow of fluid from said means for pressurizing fluid to said primary, said first alternate and said second alternate fluid jet cutting means.
 13. An apparatus for entrenching a submerged elongate means as defined in claim 12 wherein: said primary fluid jet cutting means comprises, a pair of tubular conduits operable to extend with one of said pair of conduits on one side of a vertical plane extending longitudinally through the submerged elongate means to be entrenched and with the other one of said pair of conduits on an opposite side of the vertical plane extending longitudinally through the submerged elongate means to be entrenched with said tubular conduits having a plurality of apertures therein or directing a plurality of fluid jets generally toward the fore end of said frame and into the body of water on generally opposite sides of the vertical plane extending longitudinally through the submerged elongate means to be entrenched; and said first and second alternate fluid jet cutting means each comprise, a pair of tubular conduits operable to extend with one of said pair of conduits on one side of the vertical plane extending longitudinally through the submerged elongate means to be entrenched and with the other one of said pair of conduits on an opposite side of the vertical plane extending longitudinally through the submerged elongate means to be entrenched with each of said tubular conduits having at least one but less than the total number of apertures in a conduit of said primary fluid jet cutting means for directing one or more fluid jets generally toward the fore and aft ends respectively of said frame and said pair of tubular conduits being operable to direct fluid jets into the water bed on generally opposite sides of the vertical plane extending longitudinally through the submerged elongate means to be entrenched.
 14. An apparatus for entrenching a submerged elongate means as defined in claim 13 wherein each of said tubular conduits of said first and second alternate fluid jet cutting means is provided with: a single aperture therein for directing a unitary fluid jet generally toward the water bed.
 15. An apparatus for entrenching a submerged elongate means as defined in claim 14 wherein: said means connecting said fore end of said submergible vehicle with said floating vessel is connected to said floating vessel at a fore end thereof.
 16. An apparatus for entrenching a submerged elongate means as defined in claim 14 wherein: said means connecting said fore end of said submerged vehicle with said floating vessel is connected to an aft end of said floating vessel.
 17. A method for entrenching a submerged elongate means within the bed of a body of water utilizing a floating vessel, a submergible vehicle operable to be lowered adjacent to a submerged elongate means to be entrenched, said submerged vehicle including a frame having a fore end and an aft end; a primary fluid jet cutting means connecTed to said frame and directed toward the fore end thereof; and an alternate fluid jet cutting means connected to said frame and directed toward the end thereof, comprising the steps of: lowering the submergible vehicle adjacent to a submerged elongate means to be entrenched; and selectively actuating at one time only one or the other of the primary or the alternate cutting means for eroding away a portion of the water bed positioned beneath the submerged elongate means including the step of, selectively directing a flow of high pressure fluid into the primary fluid jet cutting means comprising a pair of tubular conduits having a plurality of jetting apertures therein or, in the event a tough or hard water bed formation is encountered, into the alternate fluid jet cutting means comprising a pair of tubular conduits having a lesser number of jetting apertures therein than the primary fluid jet cutting means.
 18. A method for entrenching a submerged elongate means within the bed of a body of water utilizing a floating vessel; a submergible vehicle operable to be lowered adjacent to a submerged elongate means to be entrenched, said submerged vehicle including a frame having a fore end and an aft end; a primary cutting means connected to said frame and directed toward the fore end thereof; and an alternate cutting means connected to said frame and directed toward the fore end thereof, comprising the steps of: lowering the submergible vehicle adjacent to a submerged elongate means to be entrenched; actuating the primary cutting means for removing a portion of the water bed lying beneath the submerged elongate means; towing the submergible vehicle along the submerged elongate means to form a trench beneath the submerged elongate means within the water bed; deactuating the primary cutting means upon encountering a tough or hard formation of the water bed; actuating the alternate cutting means for eroding away the tough or hard formation of the water bed; and towing a submergible vehicle along the submerged elongate means to form a trench beneath the submerged elongate means within the tough or hard portion of the bed of the body of water.
 19. A method for entrenching a submerged elongate means as defined in claim 18 and further comprising the step of: deactuating the alternate cutting means upon passing through the tough or hard portion of the bed of the body of water; and reactuating the primary cutting means for forming a trench beneath the submerged elongate means within the bed of the body of water.
 20. A method for entrenching a submerged elongate means as defined in claim 19 wherein said steps of actuating and deactuating the primary and alternate cutting means comprises: controlling the flow of high pressure fluid into the primary cutting means comprising a pair of conduits having a plurality of jetting apertures therein or into the alternate cutting means comprising a pair of conduits having a lesser number of jetting apertures therein than the primary cutting means.
 21. A method of entrenching a submerged elongate means resting upon the bed of a body of water at an obstruction, such as a valve section, pipeline cross over, or the like, utilizing a floating vessel; a submergible vehicle operated to be lowered adjacent to a submerged elongate means to be entrenched, said submerged vehicle including a frame having a fore end and an aft end; a primary cutting means connected to said frame and directed toward the fore end thereof; a first alternate cutting means connected to said frame and directed toward the fore end thereof; and a second alternate cutting means connected to said frame and directed toward the aft end thereof; comprising the steps of: lowering the submergible vehicle adjacent to a submerged elongate means to be entrenched; actuating the primary fluid jet cutting means for eroding away a portion of the water bed lying beneath the submerged elongate means; towing the submergiblE vehicle along the submerged elongate means to form a trench beneath the submerged elongate means within the water bed; upon reaching an obstruction, deactuating the primary fluid jet cutting means; actuating the first alternate fluid jet cutting means for removing a portion of the water bed beneath the obstruction; deactuating the first alternate fluid jet cutting means; transporting the submergible vehicle over the obstruction to a location adjacent to but downstream from the obstruction; actuating the second alternate cutting means for removing the remaining portion of the water bed beneath the obstruction to form a trench within the seabed beneath the submerged elongate means at the obstruction.
 22. A method for entrenching a submerged elongate means as defined in claim 21 and further comprising the steps of: subsequent for forming a trench beneath said obstruction actuating said second alternate cutting means; actuating said primary fluid jet cutting means; and towing the submergible vehicle along the submerged elongate means to be entrenched to form a trench beneath the submerged elongate means within the bed of the body of water.
 23. A method for entrenching a submerged elongate means as defined in claim 22 wherein said steps of actuating and deactuating the primary, first and second alternate cutting means comprises: controlling the flow of high pressure fluid into the primary cutting means comprising a pair of conduits having a plurality of jetting apertures therein or into the first alternate cutting means comprising a pair of conduits having a lesser number of jetting apertures therein than the primary cutting means or into the second alternate cutting means comprising a pair of conduits having a lesser number of jetting apertures therein than the primary cutting means.
 24. A method for entrenching a pipeline riser within the bed of a body of water utilizing a floating vessel; a submergible vehicle operable to be lowered adjacent to the bottom bend to be entrenched, said submerged vehicle including a frame having a fore end and an aft end; first primary cutting means connected to said frame and directed toward the fore end thereof; second primary cutting means connected to said frame and directed toward the aft end thereof; first alternate cutting means connected to said frame and directed toward the fore end thereof; second alternate cutting means connected to said frame and directed toward the aft end thereof, comprising the steps of: lowering the submergible elongate means from a bow portion of the floating vessel to a position adjacent a bend in the pipeline riser with the aft end of the submerged vehicle in a position adjacent the pipeline riser; selectively actuating the second primary or the second alternate cutting means for forming a trench beneath the bend section of the pipeline riser within the bed of the body of water; selectively actuating the first primary cutting means or the first alternate cutting means for forming a trench beneath a submerged elongate means connected to the riser; and towing said submergible vehicle along the submerged elongate means to entrench the submerged elongate means within the bed of the body of water.
 25. A method for entrenching a pipeline riser as defined in claim 24 wherein said steps of actuating and deactuating the first and second primary and the first and second alternate cutting means comprises: controlling the flow of high pressure fluid into the first or second primary cutting means each comprising a pair of conduits having a plurality of jetting apertures therein or into the first or second alternate cutting means each comprising a pair of conduits having a lesser number of jetting apertures therein than the first or second primary cutting means. 